1. Field of the Invention
The present invention relates to a capacitor-embedded fuel cell, and more particularly to a fuel cell, which incorporates a capacitor so as to ensure a high energy density.
2. Description of the Related Art
Generally, a fuel cell is a power source for converting energy generated by virtue of electrochemical reaction between a fuel and an oxidant into electric energy, and is the primary candidate for a new generation of energy supply systems due to its environmental friendliness and high energy efficiency.
In particular, since the fuel cell has advantages of ensuring continuous use for a long time and high capacitance, it is widely believed that fuel cells will replace conventional secondary batteries used in portable electronic devices, such as mobile phones, PDAs, and notebook computers as well as ultra small actuators.
FIG. 1 is a schematic view illustrating a polymer electrolyte membrane fuel cell, which is a representative example of a conventional fuel cell.
Referring to FIG. 1, the polymer electrolyte membrane fuel cell 10 comprises a membrane electrode assembly (MEA), which includes an ion exchange membrane 11 for selectively conveying hydrogen ions, and gas diffusion electrodes 14 and 17 disposed at both sides of the ion exchange membrane 11.
The ion exchange membrane 11 consists of a solid polymeric electrolyte having a thickness of about 50˜200 μm. The gas diffusion electrodes 14 and 17 comprise catalyst layers 12 and 15, each of which is in intimate contact with the ion exchange membrane 11 and consists of carbon powders bearing a platinum-based metallic catalyst, and gas diffusion layers 13 and 16, each of which consists of a porous carbon support located on an outer surface of the catalyst layer 12 or 15. The gas diffusion electrodes 14 and 17 are provided as an anode and cathode, respectively.
The polymer electrolyte membrane fuel cell 10 further comprises separator plates 18a and 18b located at outer surfaces of the gas diffusion electrodes 14 and 17, respectively. The separator plates 18a and 18b serve to hold the membrane electrode assembly (MEA), and have passages to supply a reactant gas to the gas diffusion electrodes 14 and 17 while conveying by-products caused by reaction of the reactant gas to the outside. Although FIG. 1 shows a single MEA, the fuel cell is realized in practice as a stacked assembly having a plurality of MEAs stacked therein, and in this case, the separator plates 18a and 18b serve to electrically connect adjacent MEAs in series.
When the fuel cell 10 shown in FIG. 1 is operated, hydrogen or organic compounds such as methanol and ethanol are supplied from a separate fuel supplier (not shown) to the catalyst layer 12 through the gas diffusion layer 13 at the anode side, and oxygen or air is supplied to the catalyst layer 15 through the gas diffusion layer 16 at the cathode side, so that electrochemical reaction thereof occurs to generate electric energy together with the by-products, such as carbon dioxide, hydrogen and remaining gas, which will be discharged to the outside.
As such, since the fuel cell can be used for a long time under the condition in which the fuel is continuously supplied into the fuel cell, and since the fuel for the fuel cell is free from danger as well as inexpensive, it can be available for an ideal portable cell.
However, although the fuel cell as described above is appropriate to supply lower electric current for a long time, the fuel cell has a drawback of weakness for variation in load. This is caused by a relatively low output density thereof, and thus, when a great amount of electric current must be supplied within a short time, such as when booting a notebook computer, it is necessary to use a secondary battery or an additional capacitor along with the fuel cell.
For this purpose, a fuel cell system controller comprising the secondary battery or the capacitor has been conventionally used as an independent power source together with the fuel cell. However, in this case, there are problems in that an overall driving circuit of the fuel cell is complicated when adopting the secondary battery or the capacitor, thereby causing increase in manufacturing costs and size of the fuel cell.